The body’s natural killer cells, as their Hollywood-style name suggests, are key to the immune system. They are programmed to hunt out and destroy foreign and diseased cells. But they don’t always identify their targets. When this happens, diseases such as cancer can set in.

But a team of researchers at the Walter and Eliza Hall Institute of Medical Research have worked out what the group of highly specialised killer cells need to function at their best. It’s a protein called MCL-1.

Immunologist Nick Huntington said the protein was effectively a switch which could turn the killer cells on or off.

The discovery, outlined on Thursday in the journal Nature Communications, opens the way for new drug treatments to tame the spread of a range of diseases, including cancer.

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It could also assist patients who undergo donor stem cell or bone marrow transplants - because by manipulating the killer cell’s ‘switch’, foreign bodies such as stem cells could go unchallenged by the body’s immune system.

"It’s the only protein which does this in the cell,’’ Dr Huntington said. ‘’It needs to be turned on for the cell to survive and when it’s turned off the cell will die.’’

While aware of the existence of the MCL-1 protein and its importance at a fundamental level, scientists were previously unaware of its role in natural killer cell function. With colleagues Priyanka Sathe and Rebecca Delconte, Dr Huntington established its role.

That knowledge will prove useful for the development of new drugs to treat cancers.

Potential benefits include reduced side effects from treatment, as the killer cells only target foreign, diseased or cancerous cells, unlike chemotherapy which targets healthy cells as well.

Being able to manipulate the switch of the natural killer cells could also mean scientists can reduce the size of the cancer, once detected.

‘’Also, with surgery, you can remove a large tumour but you can’t remove or detect all the small cancers that have spread or been left behind, whereas these cells have the ability to detect them at a cellular level,’’ Dr Huntington said.

Natural killer cells are a type of white blood cell. The MCL-1 protein keeps the natural killer cells alive, allowing them to do their bit for the immune system by detecting foreign cells. Once a foreign cell such as a cancerous cell is identified, the killer cells destroys it by producing toxic granules which are thrown at the unwanted cells, puncturing the membrane and killing them.

‘’It kills them by a thousand tiny cuts,’’ he said. ‘’It just throws these molecules at the cells until the cells get holes in them and die.’’